Motor Speed – The Basics

Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz).

Our power in the US makes one full rotation from positive peak to negative peak 60 times per second or 60hz.

This means that the generators at the the power plant are running at 3600 RPM with 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute)

The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.

In a 2 pole motor it rotates all the way around every cycle, making the no load speed of 2 pole motor in the US 3600 RPM.

A 4 pole motor only goes half the way around per cycle, this makes the no load (Syncronous) RPM 1800

6 pole is 1200 no load (no slip)

8 pole is 900 no load (no slip)

So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.

An 825 RPM motor is an 8 pole motor with some allowance for slip.

A multi tap / multi speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.

This means a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than high and therefore greater slip. Low speed will have a greater winding resistance than medium and have even greater slip.

Variable speed ECM (Electronically commutated motor) motors, are actually three phase motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.

So in summary. There are three way you can change a motor speed.

  • Change the # of poles
  • Increase slip to make it slower
  • Alter the frequency (cycle rate)

— Bryan

2 comments

  1. Bill Fink says:

    Bryan, almost at the end of your tip on motor speeds from November 27th, 2016 there is a typo, the word “ten” should probably be “the”.

  2. John Shartzer says:

    Bryan,

    Thanks for this post. I have been watching a few of your videos and I listen to the podcast. I have always taken amp draw readings for direct drive indoor blower motors during a normal maintenance. I have been doing some research and now am wondering if this test is invalid. The amp draw along tells me nothing. My question is during normal maintenance test how can I test this style of motor to see if it is still in good working order? TIA. JOHN

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